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Variode Enabled Frequency-Reconfigurable Microstrip Patch Antenna with Operation Band Covering S and C Bands

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Variode Enabled Frequency-Reconfigurable Microstrip Patch

Antenna with Operation Band Covering S and C Bands

Caibiao Guo1, Lianwen Deng1, Jian Dong2, Tulin Yi3, Congwei Liao1, Shengxiang Huang1, and Heng Luo1, *

Abstract—A frequency continuous reconfigurable microstrip patch antenna with operation band covering S-band and C-band is introduced. The antenna consists of a central rectangular patch and four parasitic patches with a symmetrical structure, connected by four varactor diodes in the middle position of the edge of each patch. With help of HFSS microwave studio simulation, results have shown that, by altering the bias voltage on varactor diodes, the operated frequencies vary continuously within a wide range from 3.29 to 4.01 GHz and 5.35 to 7.00 GHz, which cover S-band and C-band. Further measurement, which verifies the simulation by reasonable agreement, has been carried out. Besides, this frequency reconfigurable antenna maintains broadside radiation and stable radiation pattern. Specifically, the gain is basically maintained at around 4.5 dBi with the working frequency increasing from 3.60 to 7.00 GHz. Compared with other frequency-reconfigurable antennas available in previous literature, the proposed antenna has advantages of a wide frequency tuning range, high frequency selectivity, simple and stable structure, low cost, and miniaturization, which make it a promising candidate as cognitive radio and future wireless communication systems.

1. INTRODUCTION

With the rapid advancement in wireless communication technologies [1–4], the demand for multi-band mobile devices [5, 6] is increasing. For example, the current wireless transceivers have to support various communication protocols such as LTE, WiMAX, and WLAN, simultaneously. Each of these technologies operates at a different frequency band. Therefore, instead of using several antennas, it is more compact and efficient to use one multi-band antenna that accommodates multiple modes. A reconfigurable antenna [7–14] is an antenna capable of dynamically modifying its frequency and radiation properties in a controlled and reversible manner in order to provide a dynamical response. By using different patch reconfigurations we obtain different results of gain, bandwidth, and efficiency.

Several approaches have been followed in regards of designing multi-band antennas. The frequency reconfigurable microstrip antenna changes structure and current distribution by loading one or more electronic control components. Therefore, in order to ensure the communication system stability [15], antennas with tunable resonant frequency while other parameters remain substantially unchanged [16] are urgently needed. With the aims at tunable operation model, switches including RF MEMS [17], and PIN diodes, varactor diodes [18], lumped element,s and electronic radio frequency (RF) switching devices [19] are commonly used to design electrical frequency reconfigurable antennas [20]. In spite of great breakthrough toward frequency-tunable property, these strategies still suffer from some drawbacks. For example, the RF MEMS’ drawbacks are lower reliability, slower response speed, higher cost than

Received 2 November 2019, Accepted 30 December 2019, Scheduled 16 January 2020 * Corresponding author: Heng Luo ([email protected]).

1 School of Physics and Electronics, Central South University, Changsha 410083, China. 2 School of Computer Science and

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PIN diodes [21] and varactor diodes. Various frequency reconfigurable microstrip patch antennas have been reported previously, because they feature miniaturization, ease of manufacture, and fast response [22, 23]. Majid et al. [24] demonstrated that the resonant frequency can be reconstructed between multiple discrete bands by adding five PIN diodes switches under a slot of the patch. Chattha et al. [4] presented a compact printed multi-band frequency reconfigurable patch antenna by inserting three PIN diodes. However, due to the turn-on and turn-off characteristics of PIN diodes, there only exist two kinds of operation model in the PIN diodes enabled system, leading to that the resonant frequency cannot be tuned continuously. Cai et al. [25] and Tateno et al. [26] demonstrated that the operating frequency of the antenna containing two varactor diodes is continuously adjustable from 2.453.55 GHz and 3.5 3.9 GHz respectively by changing the bias voltage. However, those reconfigurable antennas can be tuned in only one frequency band (S-band), and the tunable bandwidth is urgently needed to be broadened.

In this paper, a compact (33.9 ×38×1.6 mm3) frequency continuous reconfigurable microstrip antenna switching between S-band (2 4 GHz) and C-band (4 8 GHz) with coaxial single point feed is proposed. Due to the higher response speed and continuity than PIN diodes, varactor diodes are used in the simulation environment (HFSS) to achieve tunable capacitance, which is responsible for frequency reconfigurability. It seems that the two groups of varactor diodes in the symmetrical structure changed separately to stimulate two working modes, and the resonant frequency of the antenna can be continuously tuned in a wide range from 3.29 4.01 GHz and 5.35 7.00 GHz. At the same time, the whole structure of the antenna remains basically unchanged with the adjustment of capacitance so that the unidirectional radiation pattern of the antenna remains substantially constant within the tunable range, leading to a stable gain approximately around 4.5 dBi. Meanwhile, a very simple DC bias circuit is designed to control the capacitance of the varactor diodes for tuning the resonant frequency. A prototype of the designed antenna is fabricated and measured to validate simulation results. The proposed reconfigurable antenna demonstrates a reasonable agreement between the measured and simulated results. The above study provides a new research idea for broadening the tunable bandwidth of frequency-reconfigurable microstrip patch antenna.

2. RECONFIGURABLE ANTENNA DESIGN 2.1. The Geometry of the Proposed Antenna

As shown in Figure 1, the layout of the proposed antenna includes radiation patches, rectangular substrate, and the ground. Lumped elements are used in the simulation environment to achieve tunable

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capacitance, which is responsible for frequency reconfigurability. The sizes of substrate along both X andY directions are twice as large as that of the center patch. Note that four identical parasitic patches are designed around the center patch, and the feeding point is on the axis of symmetry. The structure parameters of antenna are calculated by theoretical formula of microstrip antenna, and impedance matching is realized by simulation and optimized in HFSS to make the characteristic parameters reach the maximum. Table 1 summarizes detailed dimensions of the proposed structure. SMV1247 and SMV1281 varactor diodes are selected due to their low series resistance and adequate range of junction capacitance from 0.64 8.86 pF and 0.69 13.3 pF. The equivalent circuit modes of varactor diode and its model used in the simulation software (HFSS) are illustrated in Figure 2. Source Meter is used to provide variable bias voltages for varactor diodes.

(a) (b)

Figure 2. Electrical model of varactor diode: (a) Equivalent lumped element model, (b) simulation model in HFSS.

Table 1. Optimized size of the proposed antenna.

Parameter Values (mm)

L 16.95

W 19

L1 3.39

L0 5

W0 2

2.2. Reconfigurability Mechanism and DC Bias Network

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When V1 is added (State I), the capacitances of upper and lower varactor diodes along with the x-axis direction change simultaneously, and the resonant frequency is tunable in the S-band. At the same time, V2 and V3 exert no effect on it. When V2 or V3 is added separately (State II, State III) and the other voltage remains as the highest to make the capacitance as low as possible, the tunable range of the resonant frequency is almost the same in the C-band. It is because the antenna has a symmetrical structure, contributing to the same resonant frequency, equal amplitude, and phase of State II and State III. In addition, the coaxial feeding point is just on the axis of symmetry without any connection to the ground. According to the electric field distribution on the surface of the radiation patch, the electric field of two varactor diodes along the axis of symmetry is almost unaffected by the y-axis direction.

In the actual measurement, varactor diodes are divided into three groups. In order to investigate the effect of external bias voltage on the frequency spectrum of varactor diodes, three independent DC bias voltages (namely V1, V2, and V3) are applied to patches separately. Figure 3 shows the biasing circuit for a varactor diode. Resistors and capacitors are placed to enervate the RF signal and isolate the DC in order to prevent excessive current from damaging the instrument.

Patch

DC bias voltage GND

RF choke Resistor Varactor diode

Capacitor

GND

Figure 3. DC bias circuit of the varactor diode.

3. SIMULATED AND MEASURED RESULTS

The conductive gel is used to establish an electrical connection between varactor diodes and the patch. As displayed in Figure 4, the antenna is fabricated on an FR4 Substrate (dielectric constant εr = 4.4, tanδ = 0.02) with a thickness of 1.6 mm. Simulation results are obtained using ANSYS High Frequency Structure Simulator (HFSS), and the return loss is measured by the Agilent N5230A vector network analyzer to verify the accuracy of the frequency reconfigurable antenna design.

3.1. Return Loss

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(a) (b)

Figure 4. (a) Top view of the fabricated antenna, (b) experimental equipment for return loss measurement.

3.00 3.25 3.50 3.75 4.00 4.25

-45 -40 -35 -30 -25 -20 -15 -10 -5 0

0V& 13.3pF

0.4V& 11.20pF3V&

4.80pF6V& 2.00pF

13V& 0.89pF

17V& 0.75pF

20V& 0.69pF

S11

(dB)

Frequency(GHz)

Simulation

— — Measurement

Figure 5. Dependence of return loss on DC bias voltage V1 at S-band.

5.0 5.5 6.0 6.5 7.0

-40 -35 -30 -25 -20 -15 -10 -5 0

0V& 8.86pF

0.4V& 7.22pF

0.5V& 6.17pF

1.2V& 5.13pF

3.5V& 0.83pF

2.0V& 1.88pF

2.3V& 1.48pF

3.0V& 0.95pF 1.5V&

2.96pF

5V& 0.73pF

——— — — —

8V& 0.64pF

Frequency(GHz) S11

(dB)

Simulation Measurement

Figure 6. Dependence of return loss on DC bias voltage V2 or V3 at C-band.

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(a) 3.63 GHz (b) 6.03 GHz

Figure 7. SimulatedE field distribution of the antenna at (a) 3.63 GHz and (b) 6.03 GHz.

higher range. For all tunable frequencies, the return loss is less than 10 dB. It should be pointed out that there is a slight difference between the measurement and simulation due to fabrication inaccuracies and small measurement errors. In addition, the losses from varactor diodes cannot be exactly modeled in HFSS.

3.2. Radiation Pattern and Gain

Figure 8 demonstrates simulated radiation patterns in both E-plane and H-plane at typical different center frequencies of 3.52, 3.94, 5.48, 6.03, 6.33, and 6.86 GHz. Radiation patterns maintain similar shapes throughout the entire frequency band because the structures are very similar at different states. Additionally, the maximum gain radiation direction of theE-plane andH-plane is around 0. In short, the proposed antenna is unidirectional, and the radiation pattern is stable within a wide frequency

(a) (b) (c)

(d) (e) (f)

-12 -6 0 6 0 30 60 90 120 150 180 210 240 270 300 330 -12 -6 0 6 E-Plane H-Plane Radi ation Pat tern 3.52GHz -12 -6 0 6 0 30 60 90 120 150 180 210 240 270 300 330 -12 -6 0 6 E-Plane H-Plane Radiat ion Patt ern 3.94GHz -12 -6 0 6 0 30 60 90 120 150 180 210 240 270 300 330 -12 -6 0 6 E-Plane H-Plane Radi ation Pattern 5.48GHz -12 -6 0 6 0 30 60 90 120 150 180 210 240 270 300 330 -12 -6 0 6 E-Plane H-Plane Radiation Pattern 6.03GHz -12 -6 0 6 0 30 60 90 120 150 180 210 240 270 300 330 -12 -6 0 6 E-Plane H-Plane Radiat ion Pat tern 6.33GHz -12 -6 0 6 0 30 60 90 120 150 180 210 240 270 300 330 -12 -6 0 6 E-Plane H-Plane Radiat ion Pat tern 6.86GHz

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3.2 3.4 3.6 3.8 4.0 3.0

3.5 4.0 4.5 5.0

Antenna Gain(dBi)

Frequency(GHz)

3.5 4.0 4.5 5.0

(a) (b)

State I State II, State III

5.0 5.5 6.0 6.5 7.0

Frequency(GHz)

Antenna Gain(dBi)

Figure 9. Simulated gain versus frequency of the proposed antenna.

range.

Additionally, gains at different resonant frequencies under different DC bias voltages are also displayed in Figure 9. At low frequencies, the capacitance of varactor diodes is larger when DC bias voltages are lower, and it makes the current loss increase at high capacitance levels, which leads to lower gains. However, gains are maintained at about 4.5 dBi in other tunable ranges, which are satisfactory for the monopole antenna in the intended applications. The overall performance of the proposed antenna satisfies requirements of the wireless communication system.

However, gains are maintained at about 4.5 dBi in other tunable ranges, which are satisfactory for the monopole antenna in the intended applications. The comparison of performance in our work with previous work is summarized in Table 2. It reveals that the proposed antenna has two operating bands with a smaller size and more stable gain. In addition to its compact size and four varactor diodes, the proposed antenna shows better performance in terms of wider range of reconfigurable frequency. Furthermore, the structure of the proposed antenna is simple, in low cost, and easy to fabricate.

Table 2. Performance comparison of the proposed antenna with the previous works.

Ref. Type Size (mm3) No. of Diodes Frequency Tunable Ranges Gain (dBi)

[24] disperse 50×50×3.04 5 S-band (1.983.59 GHz) 0.204.80 [25] continuous 70×70×2.0 2 S-band (2.453.55 GHz) 4.258.49 [26] continuous 80×80×2.4 2 S-band (3.50 3.90 GHz) 3.806.00 This

work continuous 33.9×38×1.6 4

S-band (3.29 4.01 GHz)

C-band (5.357.00 GHz) 4.50

4. CONCLUSION

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ACKNOWLEDGMENT

This work was supported in part by the National Key Research and Development Program of China (Grant No. 2017YFA0204600), the National Natural Science Foundation of China (Grant No. 51802352), and the Fundamental Research Funds for the Central Universities of Central South University (No. 2018zzts355).

REFERENCES

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4. Chattha, H. T., M. Hanif, X. Yang, I. E. Rana, and Q. H. Abbasi, “Frequency reconfigurable patch antenna for 4G LTE applications,” Progress In Electromagnetics Research M, Vol. 69, 1–13, 2018. 5. Chen, Y., B. Ai, Y. Niu, R. He, and Z. Han, “Resource allocation for device-to-device communications in multi-cell multi-band heterogeneous cellular networks,” IEEE Transactions on Vehicular Technology, Vol. 68, No. 5, 4760–4773, 2019.

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12. Yadav, A., M. Tewari, and R. P. Yadav, “Pixel shape ground inspired frequency reconfigurable antenna,” Progress In Electromagnetics Research C, Vol. 89, 75–85, 2019.

13. Chaouche, Y. B., F. Bouttout, M. Nedil, I. Messaoudene, and I. Benmabrouk, “A frequency reconfigurable U-shaped antenna for dual-band WIMAX/WLAN systems,” Progress In Electromagnetics Research C, Vol. 87, 63–71, 2018.

14. George, R., C. R. S. Kumar, S. Gangal, and M. Joshi, “Frequency reconfigurable pixel antenna with PIN diodes,”Progress In Electromagnetics Research Letters, Vol. 86, 59–65, 2019.

15. Shirazi, M., J. Huang, T. Li, and G. Xun, “A switchable-frequency slot-ring antenna element for designing a reconfigurable array,”IEEE Antennas &Wireless Propagation Letters, Vol. 17, No. 2, 229–233, 2017.

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17. Nafde, Y. and R. Pande, “Design and analysis of resistive series RF MEMS switches based fractal U-slot reconfigurable antenna,”Wireless Personal Communications, Vol. 97, No. 12, 1–16, 2017. 18. Nguyen-Trong, N., L. Hall, and C. Fumeaux, “A frequency- and polarization-reconfigurable

stub-loaded microstrip patch antenna,”IEEE Transactions on Antennas&Propagation, Vol. 63, No. 11, 5235–5240, 2015.

19. Ullah, S., S. A. A. Shah, M. F. Khan, S. Ullah, and J. A. Flint, “Design of a multi-band frequency reconfigurable planar monopole antenna using truncated ground plane for Wi-Fi, WLAN and WiMAX applications,” ICOSST 2014, 2014,

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21. Haraz, O. M., S. A. Alshebeili, and A. R. Sebak, “Low-cost high gain printed log-periodic dipole array antenna with dielectric lenses for V-band applications,”Microwaves Antennas&Propagation, IET, Vol. 9, No. 6, 541–552, 2015.

22. Cai, Y. M., S. Gao, Y. Yin, W. Li, and Q. Luo, “Compact-size low-profile wideband circularly polarized omnidirectional patch antenna with reconfigurable polarizations,”IEEE Transactions on Antennas & Propagation, Vol. 64, No. 5, 2016–2021, 2016.

23. Sato, S., S. Saito, and Y. Kimura, “A frequency tunable ring microstrip antenna fed by an L-probe with varactor diodes,” 2015 IEEE International Symposium on Antennas and Propagation &USNC/URSI National Radio Science Meeting, 2015,

24. Majid, H. A., M. K. A. Rahim, M. R. Hamid, N. A. Murad, and M. F. Ismail, “Frequency-reconfigurable microstrip patch-slot antenna,” IEEE Antennas & Wireless Propagation Letters, Vol. 12, No. 1921, 218–220, 2013.

25. Cai, Y., K. Li, Y. Yin, S. Gao, W. Hu, and L. Zhao, “A low-profile frequency reconfigurable grid-slotted patch antenna,”IEEE Access, Vol. 6, 36305–36312, 2018.

Figure

Figure 1. The structure of the proposed reconfigurable antenna.
Figure 2. Electrical model of varactor diode: (a) Equivalent lumped element model, (b) simulationmodel in HFSS.
Figure 3. DC bias circuit of the varactor diode.
Figure 4.(a) Top view of the fabricated antenna, (b) experimental equipment for return lossmeasurement.
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